Framers are commonly used in networking systems. Frames are used to organize the flow of information over a network line. In the case of SYNnchronous Optical Network (SONET) frames, each frame may be viewed as carrying “n” synchronous payloads envelopes (SPEs) of 810 bytes. Conceptually, a frame may be thought of as SPE1, SPE2, . . . SPEN.
For SONET frames, the time consumed by each frame corresponds to 125 μs regardless of the number of SPEs carried per frame (i.e., “n”). Furthermore, the number of SPEs carried per frame remains constant from frame to frame. Thus, the number of SPEs carried per frame is indicative of the network line speed.
For example, a SONET networking line having only one SPE per frame (i.e., n=1) corresponds to a line speed of 51.840 Mbs (i.e., 810 bytes every 125 μs). Similarly, a SONET networking line having three SPEs per frame (i.e., n=3) corresponds to a line speed of 155.52 Mbs (i.e., 2430 bytes every 125 μs), a SONET networking line having forty eight SPEs per frame (i.e., n=48) corresponds to a line speed of 2.488 Gb/s (i.e., 38880 bytes every 125 μs), etc.
One SPE per 125 μs is referred to as an STS-1 signal. Thus, a 51.840 Mbs SONET networking line carries a single STS-1 signal; a 155.52 Mbs SONET networking line carries three STS-1 signals; and a 2.488 Gb/s SONET networking line carries forty eight STS-1 signals. Typically, each STS-1 signal may be viewed as corresponding to the same SPE position across different frames. Each STS-1 signal typically has an STS identifier (STS ID) corresponding to a line and relative position in the frame. Note that if the applicable networking line is optical “OC” is typically used instead of “STS” (e.g., OC-3, OC-48, etc.).
A network line couples a pair of networking systems (e.g., switches, routers, multiplexers, gateways, etc.) so that the pair of networking systems may communicate with one another. Examples of networking lines include fiber optic or copper cable. Networking systems are typically provided with an automatic protection switch (APS) on the line card of the network element to switch signals between a working line and a separately provided protection line in the event of a line failure of the working line. This results in a change of the STS ID corresponding to a particular STS-1. The resulting change must be propagated between line cards in the network such that the protection switch for downstream line cards is dependent on the upstream line cards. For example, when a double failure occurs, the downstream node must be provided the status for both failing lines to allow flows to be properly routed.